With increasing emphasis on information in society, people are paying attention to network technologies employing an optical communications path element which includes an optical communications path in which optical signals travel. An “optical communications path element which includes an optical communications path in which optical signals travel” here refers to an optical fiber or a plastic optical fiber. Particularly, recent advances in loss reduction and bandwidth expansion of the plastic optical fiber (hereinafter, “POF”) have broaden its application range to home communications and inter-electronics communications. Measuring about 1 mm in diameter, a POF is readily coupled to an optical communications module. The use of a POF realizes an optical communications link permitting easy plug-in and pull-out of an optical fiber on an optical communications module.
A majority of conventional optical communications links using an optical fiber as a transmission medium for incoming and outgoing transmission of signal light of an equal wavelength employed all dual mode involving two optical fibers. Drawbacks of using two optical fibers included difficulty in downsizing optical communications modules and high cost of optical fibers to cover longer transmission distances. Bidirectional optical communications modules are therefore being suggested whereby optical communications is possible in all dual mode with only one optical fiber.
In such a bidirectional optical communications module, a single optical fiber is used for both incoming and outgoing transmission, so it is critical to prevent interference between outgoing and incoming light, that is, light sent out to the second party involved in communications and light sent in from that party.
Interference occurs primarily in the following four situations:
(1) An outgoing beam of light is reflected at the transmitter end face of an optical fiber, failing to enter the optical fiber. (Hereinafter, this particular reflection will be referred to as “near end reflection.”)
(2) An outgoing beam of light is reflected at the receiver end face of an optical fiber, failing to exit the optical fiber. (Hereinafter, this particular reflection will be referred to as “far end reflection.”)
(3) A beam is reflected unnecessarily in the bidirectional optical communications module located at the far end of the line. (Hereinafter, this particular reflection will be referred to as “far end module reflection.”)
(4) Light is scattered inside the bidirectional optical communications module. (Hereinafter, this phenomenon will be referred to as “internal scattering.”)
In these four cases, the scattering of light in the bidirectional optical communications module is particularly difficult to predict and hence hard to surely reduce interference caused by the internal scattering as in (4).
When used for inter-electronics communications or like purposes, an optical fiber, such as a POF, is about 1 meter long. The distance covered is relatively short and the light exiting the optical fiber may hurt the human eye. Considerations should be given to eye health risks (eye safety) in such communications. In consideration of eye safety, in such communications, the quantity of outgoing light (quantity of light exiting the optical fiber) must be set to a low value.
Typically, a semiconductor laser is used as a light source for an optical communications module. The following description will present some points to which one should pay attention in using a semiconductor laser as a light source for an optical communications module.
FIG. 18 shows a relationship between the drive current and the light output of a semiconductor laser in the range where the light output does not saturate. In the range where the light output does not saturate, the relationship between the drive current and the light output can be approximated by a broken line consisting of two straight lines. The graph in FIG. 18 indicates the light output on the vertical axis and the drive current on the horizontal axis. Comparing the two straight lines, it would be understood that the slopes of the lines differ in magnitude, although both being positive. According to FIG. 18, plotting light outputs on the vertical axis and drive currents on the horizontal axis, their relationship is represented by a straight line extending from the original point. The line however increases its slope at one particular value. In FIG. 18, the region from the original point to the value where the slope alters is indicated as B, and the region succeeding the value where the slope greatly alters is indicated as A. An extension of the straight line from the region A cuts the horizontal axis at a point indicated as Ith which represents threshold current. The regions A and B in FIG. 18 may be described approximately as being a laser oscillation region and a spontaneous emission region respectively.
If a pulse current greater than Ith is fed as a bias current, a great light output is produced when the pulse signal passes through 0 value. Therefore, the light-off ratio becomes large. Conversely, if a current less than Ith is used as a bias current, a reduction in pulse width (change in duty ratio) occurs due to an oscillation delay. Therefore, normally, the bias current is set to about Ith. If the bias current is set to around Ith, there is spontaneously emitted light even when the pulse signal is 0; therefore, a light-off ratio is determined from the ratio of the spontaneously emitted light and the light output when the pulse signal is 1. For example, to achieve a light-off ratio of 10 or more with a semiconductor laser having spontaneously emitted light of 0.3 mW, the maximum output (the output when the pulse signal is 1) needs to be set to 3 mW or more. In this manner, attention needs to be paid to the light-off ratio and changes in duty ratio.
When using an optical fiber for inter-electronics communications and like purposes, safety (eye safety) should be considered. For eye safety, the amount of outgoing beam must be set to a lower value. In using a semiconductor laser as a light source, the amount of outgoing beam can be set to a low value in several manners, one of which is to reduce the output of the semiconductor laser. However, reducing the output of the semiconductor laser makes it difficult to satisfy the light-off ratio mentioned in the above points to be considered. Further, reducing the bias current changes the duty ratio, which becomes a problem in carrying out communications. Therefore, an attempt to set the amount of outgoing beam to a low value by reducing the output of the semiconductor laser raises light-off ratio and duty ratio problems, failing to produce satisfactory results.
When using a semiconductor laser as a light source, there is another method to set the amount of outgoing beam to a low value: that is, to lower the coupling efficiency (transmission efficiency) of the outgoing beam from the semiconductor laser to the optical fiber.
The transmission efficiency can be lowered either of the two methods: (i) to lower the amount of light by the use of a filter or polarizer with a low optical transmittance, and (ii) to collect the light output of a light-emitting device and to cut off beams of light which exit at large angles using a lens called transmission lens with a small transmission lens diameter when coupling light to the optical fiber.
According to method (i), if interference occurs, interference due to far end module reflection as in (3) increases. Therefore, method (i) is difficult to apply to all dual communications using a single optical fiber. Further, other problems of method (i) include greater numbers of components. For these reasons, generally, method (ii) is used in all dual communications using a single optical fiber.
However, in method (ii), a greater proportion of light, which is cut off by the transmission lens, does not play any practical role in transmission. Therefore, if interference occurs, there is a problem of interference due to internal scattering as in (4) being likely to increase. Especially, to carry out all dual communications using a single optical fiber, the incoming beam exiting the optical fiber needs to be efficiently coupled to the light-receiving device. However, increasing reception efficiency inevitably leads to efficient reception of beams created by near end reflection and internal scattering, which in turn leads again to a problem of even more interference.
Japanese Unexamined Patent Application 11-237535/1999 (Tokukaihei 11-237535; published on Aug. 31, 1999 and Japanese Unexamined Patent Application 2001-116961 (Tokukai 2001-116961; published on Apr. 27, 2001) disclose conventional optical communications modules, which are now described immediately below.
An optical communications module described in Tokukaihei 11-237535 is now explained in reference to FIG. 19. The optical communications module is adapted with respect to the angles of outgoing beams 108, so that reflections 117 of the outgoing beams 108 do not enter a light-receiving device 105 which forms a light-receiving face. A light-emitting device 104 emits light and sends out at least part of it as the outgoing beams 108. A transmission lens 106 collects the output light from the light-emitting device 104 to form the outgoing beams 108. Having been collected, the outgoing beams 108 change their paths as they are reflected off an upward reflection mirror 110. Then, the outgoing beams 108 enter the optical fiber 102. Incoming beams 109 exiting the optical fiber 102 are coupled to the light-receiving device 105 positioned opposite to the optical fiber 102. In such an optical communications module, the reflections 117, which have exited the transmission lens 106 and reflected off the optical fiber 102, illuminates part of the light-receiving device 105 other than the light-receiving face: in other words, the outgoing beams 108 are incident on the optical fiber 102 from directions other than the directions in which the incoming beam 109 exit the optical fiber 102. By causing the outgoing beams 108 to enter in this manner, the reflections 117 illuminate part of the light-receiving device 105 other than the light-receiving face. As a result, interference due to near end reflection can be prevented from happening.
An optical communications module described in Tokukai 2001-116961 is now explained in reference to FIG. 20. The optical communications module employs a light-block plate 207. Outgoing beams 208, which are at least part of the light emitted by the light-emitting device 204, are first collected by a transmission lens 206 and then couple to an optical fiber 202. Meanwhile, the incoming beams 209 radiating from the optical fiber 202 are collected by a reception lens 224 and then coupled to the light-receiving device 205. The light-block plate 207 made of metal, etc. is disposed between a transmitting section and a receiving section. When the outgoing beams 208 are coupled to the optical fiber 202, some of the outgoing beams 208 are reflected off the optical fiber 202; the reflections are however prevented by the light-block plate 207 from being coupled to the light-receiving device 205.
According to the method disclosed in Tokukaihei 11-237535 (FIG. 19), to prevent the reflections 117 from entering the light-receiving device 105, the outgoing beams 108 need to be greatly inclined relative to the optical axis of the optical fiber 102. A greater inclination of an outgoing beam 108 to the optical axis of the optical fiber 102 results in a greater numerical aperture (NA) when the outgoing beam 108 is coupled to the optical fiber 102 and also in a deviated incident angle of the outgoing beam 108 on the optical fiber 102. In other words, the outgoing beams 108 are excited only in higher modes, not in lower modes.
As described in the foregoing, a greater numerical aperture (NA) results in a greater effect of mode dispersion in the optical fiber 102. Therefore, problems arise where transmission bandwidth is narrowed and transmission loss in the optical fiber 102 increases.
Further, coupling a deviated outgoing beam 108 to the optical fiber 102 causes following problems. If the optical fiber 102 is short, the outgoing beam 108 exits the optical fiber 102 before being stabilized and therefore the exiting light includes almost no lower modes. As a result, the light exiting the optical fiber 102 is deviated. Further, the distribution of the exiting light is like a ring with little light exiting the center of the optical fiber 102. The deviation and distribution of light affects the reception efficiency of the other module, which is a problem.
A small incident angle of the outgoing beam 108 onto the optical fiber 102 causes following problems. The light “kicked” by the transmission lens 106, that is, transmitted through the periphery of the transmission lens 106, is reflected as it hits the optical fiber 102, an optical fiber plug, etc. The reflected light is likely to cause internal scattering, which is also a problem.
According to the method disclosed in Tokukai 2001-116961 whereby the light-block plate 107 (FIG. 20) is used to separate a transmitting section and a receiving section, the part of the region of the optical fiber 102 which corresponds to the thickness of the light-block plate 107 cannot be used. Therefore, a problem arises where the reception efficiency falls. Further, this leads to a greater number of components and higher costs. The light “kicked” by the transmission lens 106, that is, transmitted through the periphery of the transmission lens 106, is reflected as it hits the optical fiber 102, the optical fiber plug, etc. The reflection lighted is likely to cause internal scattering, which is also a problem.
Especially, in an optical communications module using a POF, because of the relationship between the eye safety problems and the light-off ratio, the outgoing beams coupled to the optical fiber 102 need to be reduced by narrowing the diameter of the transmission lens 106. Reducing the outgoing beam in this manner causes increases of the light transmitted through the periphery of the transmission lens 106. As a result, conventional bidirectional optical communications modules have a problem that the light transmitted through the periphery of the transmission lens 106 becomes stray light and causes internal scattering. Note that stray light here refers to the light exiting the light-emitting device that the transmission lens 106 has prevented from being coupled to the optical fiber.
A method of reducing stray light is disclosed in Japanese Unexamined Patent Application 61-122614/1986 (Tokukaisho 61-122614; published on Jun. 10, 1986) as a method of providing a light-blocking body to a collimator lens used in a light isolator. That is, a light-blocking body is inserted between a semiconductor laser and a collimator lens, to reduce stray light produced in the lens. Further, stray light is prevented from returning to the semiconductor laser, so as to drive the semiconductor laser in a stable manner.
However, the method is to prevent self-emitted light from returning to the light source, and cannot prevent interference with a light-receiving device as with a bidirectional optical communications module. Further, the method is to reduce stray light in a lens, and cannot prevent stray light in an optical communications module or scattered light at an optical fiber plug, etc. Further, the light-emitting point of the semiconductor laser is minuscule, and it is sufficient if the method can prevent light from returning to the miniscule light-emitting point. However, in a bidirectional optical communications module, it is also necessary to separate incoming beams, and it becomes more difficult to reduce internal scattering of light. Moreover, the separation of internally scattered light and outgoing light needs to be clearly performed. Further, in cases where a light-blocking body is inserted, attentions needs to be paid also to insertion precision, managing, and attaching of the light-blocking body and degradation due to aging of the light-blocking body. This necessitates more costs and causes problems to the performance of the bidirectional optical communications module.